pinctrl: intel: merrifield: Group IO accessors in code

Consolidate IO accessors in the code to make maintenance a little bit easier
in the future.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>